The fifth pin was another power supply which was eliminated in later designs. Some later five pins are really just 4 pins with one pin not connected. You can use a four in place of a five.
Exactly correct. You cannot tell by looking if a 5 pin box is "really" a 5 pin box. You must check resistance from the "5th pin" to the other 4 to find out. I doubt ANYBODY sells 5 pin boxes, unless they are very very old stock
